In some types of thermal printing, an assembly known as a thermal printing head, that includes a linear array of heating elements, is used to heat a thermal imaging member in order to effect a change of color. The thermal printing head typically spans the thermal imaging member perpendicular to the transport direction. The thermal imaging member may be, for example, a sheet of paper coated with a thermally-sensitive composition or a donor element for dye transfer. For heating to occur with efficiency, the thermal printing head and the imaging member that is heated must be in good thermal contact. A typical practice to ensure sufficiently intimate contact is to use a platen roller located on the opposite side of the imaging member to the thermal printing head, and to apply pressure between the platen roller and the thermal printing head to bias the thermal imaging member against the thermal printing head. The platen roller often includes a deformable rubber coating that provides uniform pressure across an area referred to as the printing nip separating the platen roller from the thermal printing head.
Unfortunately, the use of a platen roller introduces a number of difficulties into the design of a thermal printer. The alignment of the line of heating elements of the thermal printing head with the axis of rotation of the platen roller is often imperfect, leading to various problems that include steering of the thermal imaging member in a direction that is not perpendicular to the line of heating elements. Eccentricity and other defects of the platen roller may introduce periodic artifacts into the printed image. Additionally, the required diameter of the platen roller introduces a constraint that may limit the compactness of the thermal printer.
There are, moreover, undesirable thermal effects that derive from the use of a platen roller that is coated with a material, such as rubber, that has poor thermal conductivity. Heat may be conducted through a thermal imaging member while it is being printed, and lead to an increase in temperature of the platen roller. When the platen roller is a poor conductor of heat, such a temperature change may be quite substantial (on the order of a few degrees Celsius). Such a temperature increase of the platen roller may lead to an undesirable change in the density of an image that is printed onto the thermal imaging member.
All these issues have led to the development of non-rotating platens such as are described, for example, in U.S. Pat. Nos. 4,327,366, 4,725,853, and 7,027,077. In these examples, pressure is provided by a spring that is independent from the platen itself in order to bias the platen (and therefore the thermal imaging member with which it is in contact) against a thermal printing head. In no case, however, is the spring described as an integral part of the platen itself.